Handling WouldChangeOwningShard requires inspecting the write errors, the current singleBatch:true + batchSize:0 for bulkWrite would not return write errors in its current form. We need to add an internal field to be sent from mongos -> mongod in order to still receive write errors even with singleBatch and batchSize:0

We do not want to expose this to users since it will have higher precedence than batchSize:0 which would be a confusing API